In a typical electrophotographic engine, images are formed by first charging a photoconductive element and then image-wise discharging that element using either an optical exposure or electronic means such as a laser scanner or light-emitting diode (LED) array. This forms an electrostatic latent image which is then developed into a visible image by passing the electrostatic latent image through an appropriate developer. The image is then transferred from the photo-conductive element to a receiver, such as paper or transparency stock, by a suitable known means such as applying an electrostatic field. The image is then permanently fixed using a suitable process such as fusing. Color images are generally produced by forming images comprising color separations on separate frames of the photoconductive element and, subsequently, transferring them, in register, to a receiver.
The process of making color images as described significantly reduces the process speed of the electrophotographic engine because the process requires two or more sequential transfers to occur. In addition, in order to register the image, it is most advantageous to wrap the receiver around a drum. This can introduce registration errors due to variations in receiver and/or drum thickness. Moreover, thick receivers cannot be wrapped around drums and it is difficult to release thin receivers from drums.
These issues were addressed, in part, by Kinoshita (U.S. Pat. No. 5,006,868), who produced two-color images by first charging a photoconductive element, comprising a conductive layer, a photogeneration layer, and a dielectric layer, by forming first and second electrically charged, oppositely charged, polarized latent images and developing said latent images using two toners of opposite polarity. The toners were then similarly charged and transferred to a print medium. As is well known, however, dielectric layers on photoconductive elements prevent the element from photodischarging, thereby creating image artifacts. Kovacs and Connell, in U.S. Pat. Nos. 5,444,463 and 5,347,303, addressed the issue of the dielectric layer by using a similar process, but they substituted a photoconductive element comprising two charge generating layers, each sensitive to a different wavelength of light. This process requires multiple scanners with specific narrowly specified wavelengths of light and photoconductive elements with very narrow absorption bands. In practice, this can be difficult to achieve and maintain over the life of the device. In addition these requirements make this process totally unsuitable for an engine with an optical exposure.
There is nothing in the prior art that teaches an electrophotographic apparatus or process capable of producing multiple-colored images that can be developed with a single rotation or pass of the photoconductive element and that also overcomes the problems of image artifacts and the necessity for exposure to multiple narrowly specified wavelengths of light.